1. Field of the Invention
The present invention relates to a video signal processing circuit, and more specifically to a video signal processing circuit which enables the reduction of noise and the enlargement of a luminance signal level so as to improve the sensitivity of a video camera that is utilized in a low-illuminating or dark site.
2. Background Information
The sensitivity of a video camera is generally dependent upon the opening degree of a lens iris and the amount of light reflected from the subject into the camera. Although the camera works with reflected light, it has become traditional in TV operations to use incident light, the illumination falling on the subject. The test of the sensitivity of the video camera necessitates a standard test patter, called "Gray Scale", which defines the brightness of white color through black color into eleven stages.
The sensitivity of the video camera, however, will be lowered when the video camera is utilized in a dark site, and will affect the performance of the resultant screen badly. Thus, there has been provided a conventional technology which increases the amplification ratio for the low-level luminance signal by wasy of an auto gain controller which provides a luminance signal of a certain raised level. But, because the auto gain controller has a low threshold relative to the amplification ratio which does not cause any noise, when incident light below a predetermined light intensity corresponding to this low threshold is inputted to the video camera, undesired noise is mixed into the video signal outputted from the video camera so that it is still insufficient to obtain good video signal from low-level incident light by utilizing only the auto gain controller.
Further, a contour compensating apparatus, which will be described in detail later in reference to the accompanied drawing of FIG. 1, has been often accommodated into the video camera to increase the sharpness of the image. The contour compensating apparatus consists of two delay lines and three adders so that it may emphasize or compensate the contours of the video input signal, of which luminance level has been in the normal operation range of the auto gain controller, for example, medium or high luminance level. However, because such a device provides low signal-to-noise ratio (S/N) relative to incident light under low illuminations, the sensitivity for the low incident light will be naturally worse.